The use of the Internet and World Wide Web (or simply the Web) is ever increasing. Web-enabled applications may make use of webpages and other user interfaces that are retrieved from servers and displayed on a user's computer to integrate web resources (e.g., content and services) within applications of the computer.
In some situations, a provider that makes web resources available to users generates revenue by reserving fixed placeholders for advertisements (ads) in the layout of user interfaces for the resources and selling the reserved space to companies or individuals. This allows an advertiser to display advertisements to users when they are displaying or interacting with various resources, such as when navigating to webpages using a browser. These advertisements, however, can result in awkward appearing webpages because the appearance of the advertisements may be inconsistent with the appearance of the remainder of the webpage. Further, reserving fixed placeholders for ads in a user interface limits flexibility and consumes space that might otherwise be used for additional content.